1. Field of the Invention
This invention is concerned with (1) delayed coking of heavy petroleum fractions and (2) disposal of petroleum sludge.
2. The Prior Art
Delayed coking has been practiced for many years. The process broadly involves thermal cracking of heavy liquid hydrocarbons to produce gas, liquid streams of various boiling ranges, and coke.
In the delayed coking process, a petroleum fraction is heated to coking temperatures and then fed into a coke drum under conditions which initiate thermal cracking. Following the cracking off of lighter constituents, polymerization of the aromatic structures occurs, depositing a porous coke mass in the drum.
In the usual application of the delayed coking process, residual oil is heated by exchanging heat with the liquid products from the process and is fed into a fractionating tower where any light products which might remain in the residual oil are distilled out. The oil is then pumped through a furnace where it is heated to the required coking temperature and discharged into the bottom of the coke drum. The oil undergoes thermal cracking and polymerization for an extended period resulting in the production of hydrocarbon vapors that leave the top of the drum and porous carbonaceous coke that remains in the drum. The vapors are then returned to the fractionation tower where they are fractionated into the desired cuts. This process is continued until the drum is substantially full of porous coke. Residual oil feed is then switched to a second parallel drum, while steam is introduced through the bottom inlet of the first drum to quench the coke. The steam strips out the oil present in the drum that was not cracked. During the early stage of steaming, the mixture of water and oil vapors continues to pass to product recovery as during the coking stage. Thereafter, the effluent from steaming is diverted to blow-down facilities in which it is condensed and transferred to settling basins where oil is skimmed from the surface of the water.
After steam cooling to about 700.degree.-750.degree. F., water is introduced to the bottom of the coke drum to complete the quench. The first portions of water are, of course, vaporized by the hot coke. The resultant steam plus oil vapor is passed to blow-down for condensation and skimming to separate oil. Water addition is continued until the drum is completely filled with water. For a period thereafter, water is introduced to overflow the drum with effluent sent to settling equipment for removal of entrained oil, etc.
The water settling system also receives water from other operations in the coker facility as later described. The clarified water so obtained provides the water for quench and for recovery of coke from the drum. Coke recovery proceeds by removal of top and bottom heads from the drum and cutting of the coke by hydraulic jets. First, a vertical pilot hole is drilled through the mass of coke to provide a channel for coke discharge through the bottom opening. Then a hydraulic jet is directed against the upper surface of the coke at a distance from the central discharge bore, cutting the coke into pieces which drop out of the coke drum through the pilot hole. The cutting jet is moved in both a circular and a vertical direction until the coke bed is completely removed.
The coke so cut from the drum appears in sizes ranging from large lumps to fine particles. To a considerable extent, the fines are separated from the larger pieces as the coke discharges into slotted bins or hopper cars with the water draining off through the slots. This dispersion of fines in water is handled to recover the fines as solid fuel, and the water returns to the system for use in quenching and cutting.
In several stages in the course of the above process, oil and coke are separated from water. A byproduct of this process is petroleum sludge--a mixture of water, oil, coke fines and other materials. Petroleum sludge is also produced in other parts of the refinery during operations such as heat exchanger and storage tank cleaning, and in the bottom of the API separator. This petroleum sludge is extremely difficult to convert into innocuous or useful (recycled) substances at reasonable cost.
Finely divided solids in liquids produce very stable dispersions and are also very effective stabilizers for liquid/liquid dispersions. Dewatering techniques are known for concentrating the sludge, but these are expensive and, at best, leave a concentrated sludge of high water content.
Petroleum refinery sludges are dispersions of oil and water having greatly different proportions of the two immiscible liquids stabilized by finely divided solids such as silt, sand, rust, high carbon content combustibles, and the like. Such dispersions are not readily susceptible to emulsion breaking techniques.
These and other sludges have been subjected to various disposal techniques at considerable expense and less than uniform satisfaction. Incineration of waste containing substantial amounts of water requires elaborate and expensive equipment. The necessary washing of incinerator stack gases has the result that the end product is still a dispersion of solids in water (i.e, a sludge).
"Land farming" is a technique for working sludge into land to permit final disposal by the slow process of bacterial action. Often, this technique is not environmentally acceptable.
Another disposal approach disclosed in U.S. Pat. No. 3,917,564 to Meyers involves mixing petroleum sludge with water and using the resulting mixture to quench the coke in the delayed coking process. While this procedure may be acceptable for producing a fuel grade coke, it is not at all clear that such a procedure would provide a green coke product suitable for providing an acceptable calcined coke product.
A very desirable process would provide an environmentally acceptable manner of disposing of petroleum sludge in a delayed coking process, while recovering the hydrocarbon liquids from the sludge, and producing a green coke suitable for making acceptable calcined coke.